Mounting systems are generally required for positioning a transducer within a housing. The mounting system positions the transducer near an opening in the housing to permit acoustic interface between the transducer and free space outside the housing. Optimum acoustic performance usually requires an acoustic seal between the mounting system and an area surrounding the opening in the housing. The acoustic seal prevents sound pressure to or from the transducer from leaking inside the housing thus degrading acoustic performance.
Optimum acoustic performance may also require that a resonant cavity be incorporated into the mounting system. The resonant cavity is a volume of space within the mounting system positioned between the transducer and the housing.
Some transducer applications require a weatherproof seal between the mounting system and the area surrounding the opening in housing to prevent weather elements from dispersing throughout the housing.
Mounting systems frequently are receptacles molded from flexible rubber. The receptacle is molded to comply with the design criteria for mechanical, acoustical and environmental performance.
Two such flexible receptacles are found in a portable cellular radio telephone (Model No. 9500XL). The application of the flexible receptacles within the radio telephone is described in Motorola Service Manual 68P81054E60-0, Part No. 68P81057E87-0. The first receptacle (Part No. 75D83456R01) is used to mount a noise cancelling microphone transducer. The second flexible receptacle (Part No. 32-82863P01) is used to mount a ringer alert transducer.
The noise cancelling microphone and ringer alert transducers each have their own independent flexible mounting receptacle. The transducers are located at opposite ends of the radio telephone. Each transducer has flexible, insulated lead wires with a connector. Each transducer is coupled to its own opening in the radio telephone's housing enabling acoustic interface to free space outside the housing. Both flexible mounting receptacles provide an acoustic and weatherproof seal.
Characteristics unique to the flexible mounting receptacle for the noise cancelling microphone transducer include: balanced chambers, an interference fit assembly design and an opening requiring silicone fill. Balanced chambers optimize the performance of the noise cancelling microphone transducer. The noise cancelling microphone detects a sound pressure level signal with both sides of its diaphragm. The signal reaches the back side of the diaphragm via holes in the back side of the microphone. The microphone will cancel the signal if the sound pressure level on both sides of the diaphragm are equal. The sound pressure level on the diaphragm is primarily determined by the distance between the source of the sound and the diaphragm. A sound pressure level signal from a far distance, such as noise, has the same pressure level on both sides of the diaphragm and thus is cancelled. A sound pressure level signal from a close distance, such as a voice signal, has a greater pressure level on the front of the diaphragm than on the back of the diaphragm thus very little of the voice signal is cancelled out. The net effect of the noise cancelling microphone is that background noise is cancelled out of the input voice signal. A noise cancelling microphone requires that the volume of the chambers on each side of the microphone transducer be equal in order to optimize its noise cancelling operation.
The microphone's receptacle is assembled into the housing using an interference fit. In addition to providing mechanical fixturing, the interference fit provides both an acoustic and weatherproof seal between the receptacle and the radio telephone's housing. Problems with the interference fit design include slow assembly time and the lack of a positive locating feature.
Silicone or similar material is typically needed to seal the opening where the flexible lead wires exit the receptacle to maintain weatherproof seal. The application of the silicone is time consuming and messy and can lead to degraded quality.
Characteristics unique to the ringer alert transducer include a resonant cavity, back pressure release and a compression fit. The resonant cavity is often molded as part of the plastic housing of the ringer alert transducer. The volume of space inside the resonant cavity is chosen to optimize the sound pressure level output of the ringer alert.
The back side of the ringer alert is open to the inside of the radio telephone housing, thus providing back pressure release. Ringer alert transducers require a volume of space behind the transducer's diaphragm to release air pressure when the diaphragm inside the ringer alert transducer deflects to produce a sound pressure level signal in front of the diaphragm.
The ringer alert is assembled within the aforementioned radio telephone using a bracket held by two screws and a flexible molded receptacle. The bracket positions the ringer alert and compresses the flexible molded receptacle between the ringer alert transducer and the housing to provide an acoustic and weatherproof seal.
Although the transducer mounting systems, as described above, meet the general requirements of the application, improved cost, quality, and assembly advantages may be realized using a new mounting system as described below.